1. Field of the Invention
The present invention relates to an aluminum foil which is used as the electrodes in electrolytic condensers, and more particularly which is used as anodes in the middle- or high-voltage electrolytic condensers, and further relates to a method of making the aluminum foil.
2. Description of Prior Art
Many research works have been conducted in order that the density of the etching pits may be increased, wherein the etching pits are formed by electrically or electrochemically etching the surfaces of the aluminum foils for use as the electrolytic condenser electrodes so as to increase their effective surface area and thus their capacitance per unit area. Among such recent works, a study reported on the Japanese Patent Publication Sho. 62-42370 has developed a new technology in which the foil surface layer is enriched with at least one of Pb (lead), Bi (bismuth) and In (indium). This technology is based on such a knowledge that composition and fine structure of the aluminum foil surface layer is a key factor determining the etching pit density. In detail, compounds of at least one of Pb, Bi and In are applied to the foil surface layers, and thermal diffusion process is carried out at temperatures above the melting points of these metals, and annealing process may subsequently be conducted according to the common practice, if necessary.